A review paper by L. R. Dawson entitled "Liquid phase epitaxy (LPE) techniques for compound semiconductor growth" published 1982 in SPIE Vol. 323 at 138 is a good summary of the current status of LPE. The abstract of this paper summarizes LPE as follows:
"Liquid phase epitaxy (LPE) is a method of crystal growth well suited to the preparation of a wide range of compound semiconductor materials including GaAs, AlAs, GaP, InP, and GaSb, as well as their ternary and quaternary alloys. The advantages of LPE over other solution growth methods are substantial, primarily in material purity, doping flexibility and dimensional control. It has particular advantages in achieving the complex multilayer structures required for many interesting optical devices, such as injection lasers, light emitting diodes, and photodetectors. LPE has appeared in many configurations in recent years, with the dominant variation at present being the sliding boat method. A good understanding of the capabilities of this method can be obtained by studying the growth of GaAs-Al.sub.x Ga.sub.1-x As heterostructures".
In manufacturing the above mentioned multilayered structures the control and uniformity of layer thickness are of paramount importance. In particular, uniformity from substrate to substrate and batch to batch is critical.
In addition to thickness uniformity, purity and freedom from contamination resulting from so-called cross-contamination of the source melt are also important.
The most commonly used LPE boat, a machined graphite carrier for substrate and source melt, dates back to U.S. Pat. No. 3,565,702 issued 1971 to H. Nelson.
Variations and improvements on the basic boat of Nelson are discussed in two papers. The first by S. Y. Leung and N. E. Schumaker deals with "Slider induced convection in horizontal liquid phase expitaxy (LPE) system", is published 1982 in SPIE Vol. 323, p. 156 and examines the conventional, the confined and the baffled melt holders, finding the last two equally preferable to the first.
The second paper by K. Takahei and H. Nagai entitled "LPE GROWTH OF 1.5-1.6 .mu.m In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y CRYSTALS BY A MODIFIED SOURCE-SEED METHOD" published 1981 in the Journal of Crystal Growth 51, p. 541 describes a process in which the top of the source melt is removed prior to epitaxial growth. The paper concludes:
"1.5-1.6 .mu.m wavelength region InP/InGaAsP/InP double heterostructure crystals were grown at a relatively low temperature (592.degree. C.) in order to prevent the melt-back of the quaternary layer. A high reproducibility of quaternary layer properties, such as layer thickness, wavelength, and lattice matching with InP, has been accomplished even at such low temperature by utilizing a modified source-seed (MSS) method. Relations between LPE growth conditions of the MSS method and the properties of the quaternary layers were formulated in a convenient way for use in an actual practice of LPE growth, and double heterostructure crystals with quaternary layers of various wavelengths, which lattice match to InP, have been grown with high controllability. Such a technique is very useful for fabricating semiconductor lasers of various wavelength in the 1.5-1.6 .mu.m wavelength region, where an optical communication system with wavelength multiplexing is expected".